A common type of interior decorative trim panel or cover used to cover and hide the air bag in a motor vehicle comprises a thin elastic plastic skin having a non-glaring grained outer surface, a rigid retainer or substrate and an intermediate layer of soft plastic foam between the skin and substrate in some cases the foam is omitted. The present invention relates to those covers wherein the skin is formed of a thermoplastic polymer or thermosetting resin and is provided with an invisible tear seam, and the substrate includes one or more doors that are impacted by the inflating air bag and press against the cover to separate the tear seam and then swing outward to form an opening in the cover for deployment of the air bag into a protective position in the passenger space. These tear seams are provided in various configurations or patterns with the most common having a C, H, U, or X-shape and wherein the pattern determines the number of doors required in the substrate,
It is desirable that the tear seams and thereby the presence of the air bag be hidden from view for various reasons and heretofore, this has been accomplished in several different ways. Such a tear seam is commonly referred to as an "invisible tear seam". One way of providing such an invisible tear seam is by forming a tear seam defining groove or series of depressions in the backside of the skin. This leaves a thin and thereby weakened section at the outer or appearance side of the skin that defines the tear seam without outwardly revealing its presence during normal view by an ordinary vehicle occupant. Examples of such invisible air bag cover tear seams are disclosed in U.S. Pat. Nos. 5,072,967; 5,082,310; 5,316,822, 5,348,339 and 5,632,914. The tear seam may be formed in the backside of the skin in various ways as is well known by those skilled in the art.
While suitable for their intended purposes each of the proposals for weakening the seam require precise and/or multiple processing sequences that can add to the cost of manufacture of the air bag cover. Further, it has been found that under certain conditions, the location of an invisible tear seam on the backside of the skin may "read through " and be detectable on the frontside of the skin.
In the case of laser scoring, U.S. Pat. No. 5,632,914 patent assigned to the owner of the present invention, discloses an arrangement that requires a precision adjustment of very small holes that completely penetrate a shell. While the arrangement is undetectable by normal viewing, it represents an interruption in the surface of the shell or skin cover of the air bag. Furthermore, the very small nature of the holes combined with a high quantity to facilitate adequate weakening results in an increase in equipment precision and cycle time, with both attributes resulting in increased manufacturing costs.